/Engine/Source/Runtime/Core/Public/Math/UnrealMathUtility.h
Returns closest point on a tetrahedron to a point. The idea is to identify the halfplanes that the point is in relative to each face of the tetrahedron
Point on tetrahedron ABCD closest to given point
static FVector ClosestPointOnTetrahedronToPoint ( const FVector & Point, const FVector & A, const FVector & B, const FVector & C, const FVector & D )